1. Field of the Invention
This invention relates generally to machinery and methods for automated placement of fiber reinforcement in manufacture of composite materials; and more particularly to such placement accurately on surfaces of arbitrary three-dimensional contour--including planar surfaces.
2. Prior Art
During the past three decades, high-performance continuous fiber-reinforced polymeric composites have gained a degree of acceptance as a result of their attractive ratio of strength to weight. Composites are particularly well developed in the aerospace, transportation, sporting-goods, and machinery industries.
Primarily because of high material and manufacturing costs, however, use of reinforced composites has heretofore remained an exotic option. With isolated exceptions, composites have been limited in use to a relatively few extremely demanding applications in which cost is a secondary consideration.
Composites differ from traditional engineering materials in that a second material phase, most often a filamentous reinforcement, has been added to obtain specific material characteristics not available from a single unmodified material. The second phase may be added to provide strength, stiffness, toughness or other properties.
By combining filaments with thermoset or thermoplastic polymeric resin, a structure can be fabricated with properties customized to the intended use. Structural properties of such a composite are in part determined by the controlled distribution of one of the materials, the filamentous reinforcement, in a continuous phase of the second, the matrix.
One example of the thermosetting composite materials is epoxy resin reinforced with glass filament--available in forms known as "continuous tow," unidirectional "prepreg," or woven material. A tow (pronounced as in "toe") is a collection of a usually large number of mutually aligned filaments, and is usually assumed to have a generally round cross-section.
Unidirectional prepreg (resin-preimpregnated tape) is the same as a tow except that it usually has a generally flattened cross-section. For simplicity and conciseness in this document, however, the term "tow" is hereby defined to encompass flattened unidirectional prepreg as well as material of generally round cross-section, and also other forms to be mentioned shortly.
Prepreg manufacturing processes include Pulling groups of filaments together through resin baths, so that the resin envelops and generally occupies the spaces between the filaments. In such processes--as well as in some textile-handling processes--shallow comb structures are sometimes or often used as guides to maintain the fibers parallel and in proper lateral spacing, and to prevent their twisting.
An example of thermoplastic composite materials is polyetheretherketone (PEEK) resin reinforced with carbon fiber and available as comingled yarn, unidirectional prepreg or woven material. (The term "fiber" is often used interchangeably with the word "filament," and for present purposes they may be considered equivalent.) Other material forms are available for special applications.
To enable these composite materials to be used in various applications, appropriate fiber-placement techniques are required. Processes now in use to fabricate composite structures from the forms of composite materials described above include (1) manual layup, (2) spray-up, (3) braiding, (4) so-called "pultrusion", (5) filament winding, and (6) tape laying.
Each of these techniques will be discussed below. Implementing these methods, and also blurring some of their definitions, are certain automated devices of the prior art which will also be described below.
For the most part the present invention provides an alternative to these techniques; and for many kinds of projects it is an advancement over all of them. As will be seen, however, the invention also has some potential for use in complementary combinations with certain of these known processes.
In four of the six fiber-placement processes, fibers are placed either on a substrate which will later remain part of the finished article, or on a tool from which the finished article is later separated. The exceptions are braiding and pultrusion, which are used to fashion structural elements for subsequent assembly into more elaborate structures.
After fiber placement, a part fabricated of composite materials is commonly heated and consolidated, often in an oven or autoclave. This process is usually called "curing" the material.
(1) Manual layup in principle may use almost any form of material, but is usually practiced with woven material or wide tape. It is very labor intensive, relatively slow, and not very precise (reproducible) in terms of either positioning or structural qualities--e.g., uniformity of load bearing.
Manual layup does provide maximum versatility, and therefore is extensively used in forming compound shapes. It is also widely used when programming or other elaborate make-ready work for automated fiber placement is not economic, as in one-of-a-kind or short-run items.
(2) Spray-up is an inexpensive manufacturing method used where weight and high directional strength are not critical. A pneumatic device similar to a paint sprayer applies a fiber-and-resin mixture to a substrate or tool.
In spray-up the fibers are necessarily rather short, and are in a generally random distribution of positions and orientations. Finished articles accordingly have low, nondirectional tensile strength, and rough, abrasive surfaces.
(3) Braiding is essentially a weaving process. Fibers pass over and under one another to form a two-dimensional netlike array that is strong in two directions. To a limited extent this braided array of fibers can be shaped into three-dimensional articles before cure.
(4) Pultrusion is like extrusion in some ways: a material, while in a formable condition, is forced through a die to create an extended structural member having a particular structurally useful cross-section. In pultrusion, however, the material before passing through the die is not isotropic and so cannot simply be squeezed hydraulically out of a homogeneous melt.
Rather the object of pultrusion is to incorporate in the finished member longitudinal filaments that impart extremely high tensile strength longitudinally. These filaments accordingly must be suitably oriented within the material before passage through the die.
Thus in pultrusion the filaments and resin together are pulled, rather than squeezed, out through the die. Resulting structural units are cut to suitable lengths or, if thin and flexible enough, rolled onto a storage spool.
Pultrusion is capable of forming relatively large, well-defined members such as cylinders, "I" beams, and so forth. (It could also be used, however, to make specially shaped tows for use with the present invention, in ways which the disclosure of the invention will make clear.)
(5) Filament winding is sometimes considered to be the same as "precision fiber placement." More strictly speaking, filament winding is limited to winding of filaments or tows around a rotating work piece or tool, called a "mandrel," along a circuitous path. Most successfully such a path is diametral or near-diametral, so that the filaments are uniformly tensioned and well anchored against the mandrel until cured.
In fiber or filament placement, by contrast, theoretically any predetermined orderly path may be used in creating any given shape or form. As a practical matter, no apparatus for accomplishing such arbitrary placement has heretofore been available.
(6) The last-mentioned process, tape laying, has been used to fabricate large panels, either three-dimensionally contoured or flat, typically from three-inch-wide, five-mil-thick unidirectional prepreg tape. Several patents have been issued for automatic or semiautomatic equipment to speed up or refine this process.
To form a three-dimensional surface from pieces of tape that are essentially planar is a touchy exercise in approximations and compromises. The process is related to the well-known projection problem of making a two-dimensional map of a three-dimensional curved surface, such as the surface of the earth.
Few patents appear to address directly this problem of forming compound shapes from filamentous composites. One such document is U.S. Pat. No. 4,541,886 of Marlow, Wiltshire and Hulme.
That patent, assigned to British Aerospace PLC, describes a huge automatic machine for laying planar tape, one piece at a time, to make large panels--apparently for aircraft wings or other parts. With the Marlow et al. machine, the task is performed in two steps.
Each length of tape is first individually prelaid on a table, cut in plan, checked, and then transferred by a pair of vacuum-pickup rollers to a final position. The process is preprogrammed to bring each piece of tape near tangency everywhere with the three-dimensional contour beneath it--within the capacity of the tape to distort out-of-plane.
Perhaps the largest machine of this general type is disclosed by August and Huber, assignors to Grumman Aerospace Corporation, in U.S. Pat. No. 4,133,711. Another patent describing a similar device, but said to be relatively light in construction and therefore in shop requirements, is U.S. Pat. No. 4,591,402--issued to Evans and Murray, and assigned to LTV Aerospace and Defense Company. Each of these machines is like the Marlow unit, in laying one piece of planar tape at a time, in a two-stage process, for large panels such as aircraft parts.
Evans et al. describe their device as providing "complex position modifications required for X and Z-axis adjustment, to follow three dimensional mold contours". Their discussion, however, does not solidly confront the projection problem--i.e., the matter of how to make three-dimensionally contoured parts from planar tape.
From this omission it may be assumed that their device like Marlow's is intended for fabrication of aircraft parts that are so large, and whose curvature is so slight, that tape of typical dimensions can be laid down with negligible contour error. For general fabricating work outside the aircraft industry, these simplifying assumptions are seldom applicable.
Another related apparatus is disclosed by Wisbey in U.S. Pat. No. 4,557,790, assigned to Cincinnati Milacron Incorporated. This machine uses many very narrow tows to reduce cutting waste.
Although the use of such narrow tows might also help with the three-dimensional projection problem, Wisbey does not seem to mention this effect. His machine assembles a wide tape from many differential-strip tows, and then transfers the entire assemblage to the workpiece.
Still another device of the same general sort but considerably earlier is U.S. Pat. No. 3,775,219 of Karlson and Hardesty. The assignee is Goldsworthy Engineering Incorporated, of Torrance, Calif.
This machine lays planar tape three inches wide in a single step--that is, directly from a supply roll to a workpiece, without precutting on a work table. The disclosure illustrates a three-dimensional workpiece, but, again, does not seem to address the projection problem.
Other large flatbed machines related to this field are disclosed in U.S. Pat. No. 3,345,230 to McClean, U.S. Pat. No. 3,689,349 and U.S. Pat. No. 3,711,354 to Burger, and U.S. Pat. No. 4,292,108 to Weiss, Hudson and Dowell. These all make planar crosslaid fiber structures.
The McClean and Burger devices make planar crosslaid fiber webs from which articles can then be fabricated. The Weiss et al. machine forms large structural members for aircraft and the like.
Innovations of the Weiss machine appear to lie in the area of trimming complicated edge shapes. Whether it can make contoured structures is not completely clear: the device is said to be used for making "laminated structural members"; and these seem to be "portions of the horizontal and vertical stabilizer skins".
Finally, U.S. Pat. Nos. 4,382,836 and 4,560,433 issued to Frank, and assigned to The Boeing Company, deal with linearly reciprocating applicator heads for laying tape to form articles such as helicopter rotor blades. Each of these applicator heads dispenses and compacts a single planar tape onto a mandrel or other working surface.
The machine operates in both directions, and at the end of each pass either shears the tape or makes an antistress loop as the operator designates.
Frank indicates that his "tape applicator head . . . can have as many degrees of freedom of movement relative to the working surface . . . as desired. The manner in which this is accomplished is not part of the invention. It is dictated primarily by the shape of the structure to be constructed. Preferably, the tape applicator head . . . is mounted for displacement along three mutually orthogonal axes relative to the work surface . . . . In addition, the head may be rotatable relative to any or all of these three axes."
Limitations of tape-laying apparatus thus include extremely high capital cost and a relatively high degree of specialization of the equipment to particular manufacturing projects, or at least project types. When used in aerospace work, for example, these inordinately expensive machines may pass into extended periods of disuse after completion of particular manufacturing contracts.
Obviously the economics of such overspecialized fabrication equipment are not ideal. These machinery-based tape-laying methods are at the opposite end of the versatility spectrum (as well as the labor-intensiveness spectrum) from the manual-layup approach discussed earlier.
Neither method is adequately economic for general-run fabrication. It can accordingly be understood why manufacture by reinforced composites has heretofore been mostly constrained to certain very valuable products, extremely demanding in strength-to-weight ratio, for which high cost is a small obstacle.
The foregoing discussion of known methods demonstrates that the prior art has made very limited provision for automatically fabricating three-dimensionally contoured surfaces. Aside from manual layup, such surfaces are made in only two situations: diametrally or near-diametrally wound figures of revolution, and very large constructions in which multiple segments of planar tape are individually applied one at a time to build up a desired surface.
For arbitrarily contoured articles of more modest size, neither of these methods will normally be useful. Near-diametral winding clearly operates for only a very limited few kinds of shapes.
Individual application of planar tape segments has three very severe drawbacks for arbitrarily contoured small items. First, in scaling down the automated techniques used for aircraft wings and the like, one rapidly reaches very narrow tape widths--and microscopic tape thicknesses--that would be cumbersome and difficult to handle.
Secondly, also in adapting aircraft-manufacturing methods, one finds that general fabrication of small articles will often entail surfaces far more strongly contoured than aircraft wings and the like. In combination with the problems of scale just mentioned, the necessity for much sharper curves in general fabrication creates very severe questions of feasibility.
Thirdly, material-handling protocols that involve individual handling of incremental strips of material twice may be economic for making multimillion-dollar aircraft, or even hang gliders and top-of-the-line tennis rackets, but do not appear practical for more mundane articles.
Furthermore, although some high-performance articles do justify highcost techniques, even for those articles economy is generally welcome. For example, there is room for cost improvements in short-run production of special aircraft, and in analogous situations in most of the other fields where composites have already become established.
Finally, while the foregoing discussion has focused on forming arbitrary contours, even procedures for forming complicated flat shapes on flat mandrels will bear improvement. Tape-laying may be cumbersome and uneconomic in this context because of desired variations in width, reinforcement direction, etc.
Thus the prior art has not provided adequately for reinforced-composite fabrication of arbitrarily contoured articles, particularly small articles and articles with compound curves. Even laying down a complicated shape on a planar mandrel has been difficult heretofore.